Firefighters said the fire could easily have set a car and the adjoining house alight and urged Islanders to dispose of ashes properly.
The residents of the house awoke at around 6 am to find their garage filled with smoke. They called 999 after evacuating the property.
Two fire engines, one from Fire and Rescue Service headquarters in St Helier and the other from the Western Fire Station at Red Houses, attended the incident. Firefighters wearing breathing apparatus because of the smoke used a high-pressure hose to extinguish the fire.
A fire service spokesman said: ‘An investigation established that discarded ash and coals, removed from the fireplace the day before, had been placed on top of several bags of house coal stored in the garage.
‘Over a period of many hours, residual heat had melted through the plastic sacks and ignited the coal within. The coal was removed from the garage and fully extinguished and an inspection of the damage conducted.’
Fortunately, no one was hurt and there was no major damage to the property.
The spokesman added: ‘This incident demonstrates how discarded coal and ash from household fire grates needs to be disposed of correctly in order to prevent fires.
‘In this instance the damage was relatively minor, although the fire could have easily set fire to the car parked in the garage and potentially to the adjoining house.
‘Jersey Fire and Rescue advise that ash and used coals removed from the fire grate should be stored in a metal bucket, or similar, and kept outside away from combustible surfaces and items, until thoroughly cooled.’